1. Field of the Invention
The present invention is broadly concerned with improved induction heatable articles, such as food-heating pans, which are designed to permit differential magnetic induction Joule heating of respective portions of the articles. More particularly, the invention is concerned with such articles, induction heating assemblies making use of the articles, and corresponding methods. Preferably, the induction heatable articles are formed of synthetic resin material and include one or more susceptor coils having a plurality of zones each adjacent a different portion of the articles and capable of providing respective, different levels of Joule heating to the different portions under the influence of an applied alternating magnetic induction field. In another aspect of the invention, a highly energy efficient, induction heated food heating/warming table is provided with various sizes of pans which can be individually and differentially heated at user-selected temperature levels.
2. Description of the Prior Art
Vessels and pans have been provided in the past with surface walls actively maintained at desired temperatures so as to maintain the contents of the vessels or pans at desired temperature levels. For example, water tanks for brewing coffee typically employ a resistive heating element wrapped around an upright cylindrical stainless steel tank wall, and also have a similar element adjacent the bottom wall of the tank. Electrical current flowing through the resistive elements heat the latter, with temperature regulation being provided via temperature feedback using a thermal sensor attached to an outer tank wall or disposed in the water.
In addition, food pans employed in steam wells for food service applications, whether stainless steel or of polymer construction, are heated by steam on all four sides and the bottom thereof so as to maintain food therein at desired, safe temperatures. The steam generator system of the well may be regulated by temperature feedback from one or more temperature sensors in the well. Steam wells are capable of maintaining all surfaces of a pan in the well at nearly the same temperature because the steam contacts all pan surfaces and condenses on the surfaces as a function of the temperature of that surface area. In essence, with steam heating, each unit surface area of the pans acts as its own thermal sensor for feedback to the steam heat source.
However, conventional steam tables suffer from a number of deficiencies. First, these units require considerable energy inputs to create and maintain the heating steam. This energy inefficiency is compounded because of the fact that the HVAC systems of the buildings housing the steam tables must be operated to counteract the heat and humidity generated by the steam tables. Furthermore, the steam tables containing warm/hot water can be contaminated with undesirable microorganisms which can in turn contaminate the food being served.
Vessels or pans heated by induced current on multiple surface walls are also known in the prior art. For example, U.S. Pat. No. 5,954,984 discloses vessels employing wrapped coil heating elements which are heated by Joule heating of the coil elements via induction heating using a work coil. An induction heater with an impedance detection capability is used to trigger the magnetic field on and off in response to the load (i.e., the vessel) impedance which is itself a function of the state of an associated temperature switch. Further, U.S. Pat. No. 6,504,135 describes induction heatable coil heating elements with a switchable element, such as a thermal switch, allowing the coil element to interact with the induction heating system described in the '984 patent.
CookTek Induction Systems, LLC of Chicago, Ill., has commercialized a waterless food heating/warming table system (referred to as the “SinAqua” system) wherein stainless steel pans are induction heated. However, there is no means of differentially heating different portions of the stainless steel food-holding pans, i.e., the entirety of the pans is heated by appropriately positioned induction work coils.
Thus, while induction heating of pans is well known, a number of problems remain. Most importantly, prior induction heating systems have not provided effective means for differentially heating respective portions of a pan or vessel to different temperatures or over different time periods. This is especially the case in connection with food heating/warming table systems having different and differently sized pans holding different kinds of foods. Optimally, such pans should be individually heated with, for example, the principal heating through the bottom walls of the pans, while also providing secondary heating around the sides and ends of the pans.